The 146-foot long, 416 ton towboat Mary L with a tow of eleven empty barges was forced to run aground as it approached the Barkley Dam on the Cumberland River causing it to strike one of the piers of the Eureka Highway Bridge.   An inspection found no damage to the bridge.   The cause of the grounding was due to the high volume of water being discharged from the Barkley Dam.   No injuries or damage to the barges.